Active robust structural controls have been utilized in the control of aerospace structures for many years but they have only been recently investigated in the context of control for civil engineering structures. The results of an investigation of the utilization of these methods on building-like structures are presented in this paper. The closed-loop systems take into account the limited available actuation force and are inherently insensitive to parameter variations and modeling uncertainties. Independent modal-space control (IMSC) is a structural control technique where the multi-input-multi-output configuration-space system is transformed into a set of uncoupled single-input-single-output modal-space systems. A modal controller is designed for each modal-space system and the set of modal controllers is transformed back into configuration-space. By combining IMSC with robust control techniques such as LQG/LTR or H(infinity ), a robust structural control design technique is proposed in this paper. Robust IMSC techniques are employed for control of seismic structures where a small number of actuators are used to control the first few modes of the structure. We have designed and implemented robust IMSC controllers on an experimental building-like structure. This structure utilizes torque motor driven active tendons as actuators and rests on a shaking table which is capable of providing one dimensional base excitation similar to earthquake ground motion. A three input-three output model of the structure, including the torque motor actuators, was developed using experimental data. The experimental structural identification technique, based on standard modal analysis methods, provides the mathematical model that describes the behavior of the structure. An H(infinity ) based IMSC controller has been designed and implemented on this structure using a dSPACE control development system. The results show that the performance of the system is satisfactory in the presence of unmodeled dynamics, parameter variations, and disturbance inputs.

The objective of this paper is to evaluate the relative performance of several Bayesian distance-based pattern recognition models and two non-Bayesian models for non-destructive damage detection (NDD). A theory of damage localization, which yields information on the location of the damage directly from changes in mode shapes, is formulated. Next, the application of pattern recognition for NDD is established. Expressions for pattern classification using discriminate functions based on Bayes' Rule, Neyman-Pearson criteria, and neural networks are generated. A set of criteria for the evaluation of the pattern recognition models is then established. Damage localization is applied to a finite element mode of a structure which contains simulated damage at various locations using the pattern recognition and neural network models. Finally, the evaluation of the pattern recognition models is carried out using the established criteria.

In this paper nondestructive damage detection (NDD) in large/complex structures is demonstrated via simulating vibration monitoring of such structures. The theory of NDD for truss type structures is formulated. To examine the feasibility of the theory, a finite element model of a 3-D truss structure, which consists of sixteen bays and includes 246 elements, is developed to simulate damage. Four damage cases are simulated numerically. The cases range from the structure being damaged in one location to the structure being damaged in three locations. For the given modal parameters, this study reveals very good results for small aounts of damage as well as large damage.

The evolution of the vertical displacements is considered to be a good criterion to assess the condition of a bridge. Most embedded deformation sensors (e.g. inductive sensors, optical fiber sensor...) give the relative displacement between two points in the same structure. Therefore, the vertical displacements of a bridge in an earth-bound coordinate system are not directly supplied by the deformation sensors. This paper presents an algorithm to determine the vertical displacement from internal deformation measurements in the bridge. It is shown that 6n deformation sensors are necessary to draw the exact vertical displacement of a bridge uniformly loaded on n plus 1 supports. A load test has been carried out on a 6 m long timber beam on 3 supports, instrumented with 12 fiber optic deformation sensors. The vertical displacements calculated by the mathematical model on the base of the horizontal measurements are found to be in good agrement with the ones obtained by external dial gages.

The bond strength and contact electrical resistivity between steel rebar and concrete were found to be linearly related, due to an interfacial phase of high volume resistivity that helps the boding. Acetone washing of the rebar increases the bond strength slightly and decreases the contact resistivity slightly, but does not affect the linear correlation, which provides a nondestructive method for bond strength assessment.

Self-monitoring of slight fatigue damage was demonstrated in cement mortar containing short carbon fibers (0.24 vol.%), as damage (occurring in the first less than or equal to 10% of the tensile or compressive fatigue life) caused the volume electrical resistivity to decrease irreversibly by up to 2%. The greater the stress amplitude, the greater the damage, the greater the resistivity decrease and the greater the number of stress cycles for which the resistivity decrease monotonically occurred. The resistivity decrease is attributed to the damage of the cement matrix separating adjacent fibers at their junction.

The knowledge of loads applied to bridges has to be enhanced in order to actualize national and international codes, like Eurocodes. The nature of traffic loads is extremely complex including such phenomena as dynamic effects, random distribution of damping techniques over the actual trucks, multiple non-linear visco-elastic links in mechanical description of a given truck. For all these reasons, a system of monitoring bridges has been preferred to an unrigorous modelling, in order to get a statistical knowledge of the traffic loads applied to the bridge over large periods. This knowledge under the form of histograms will be useful in order to evaluate extreme load effects and fatigue load effects over the lifetime of the bridge. To achieve these goals, a data acquisition system based on smart sensors extracting and classifying extrema in the traffic loads signal has been developed. At each measurement site a small microsystem is dedicated to the tasks of signal conditioning and sampling, calculation and communication. Each smart sensor can communicate through a numerical data link with its neighbors or with a PC based system controller. In this paper an outline of the problem, the proposed solution based on the smart sensor paradigm, and the results which have been obtained are presented.

The construction phase of a building is one in which the structure is rather vulnerable to damage and/or collapse. The potential for human and economic loss, as well as the potential for reduced construction costs, presents an opportunity for developing construction structural load and response monitoring systems that can possibly be effective in reducing the frequency of collapses and/or damage. This paper presents the results of a study in which such systems were developed and applied to slab-style concrete building construction. The shoring systems that are used to support fresh concrete floor slabs are instrumented with strain gage based load cells to measure the loads during construction operations. The loads on the shores have been measured during the construction of three different buildings. This information is being used for construction procedure code development. The next phase of this research is to apply the sensing system so that it can provide an early warning for potential collapses of the partially- built structure. Mechanical models of the structural system indicate that load monitoring of individual shoring members is probably insufficient because the major structural collapses are due to a global instability. It appears that a combination of strategically-placed load, temperature and deflection monitoring instrumentation combined with a real-time analysis of the data may be necessary. The design and use of such systems are discussed.

This paper discusses efforts related to the development of smart bolts to be used as structural attachment fixtures. The work has been directed at meeting the high-strength application requirements for the aircraft industry and selected applications within the construction industry. The bolts are fabricated from metastable austenitic steel materials which progressively and irreversibly transform from a nonferromagnetic, austenitic parent phase to a thermodynamically stable, ferromagnetic martensitic phase as a function of applied strain. Thus, the ferromagnetic response of a bolt in service can be used to indicate the degree of inelastic deformation, i.e., the peak strain, that the bolt has experienced up to that particular point. A combination of bolt alloy chemistry and thermomechanical treatment has been utilized to produce a sharp ferromagnetic response with respect to small plastic deformation strains. The extreme strength of the materials allows for a centrally drilled access hole for the placement of sensitive Hall sensor probes to detect any inelastic material behavior along the length of the bolt without removal from the structure. A discussion of the smart materials behavior of the bolts will be followed by a presentation of recent test results which illustrate the structural bolt monitoring technique with some possible applications.

Currently there exists sufficient sensor and computational technology to allow an individual to instrument full scale structures and record numerous input channel records. In past practice significant effort was on refining the mechanical task of data acquisition. While the aforementioned venue has been thoroughly explored and expanded, the more labor intensive task of data reduction, analysis, and interpretation remains largely unchanged. Such analysis tasks are most commonly performed by technical personnel with the aid of a computer. Provided that the data analysis procedure can be clearly defined, this procedure is a fitting candidate for automation. Given the current advances in desktop workstation flexibility, the possibility of an automated intelligent signal processing and health monitoring system for an instrumented structure (located at a remote site) is well within our grasp. With the addition of current Internet interaction technology, this system would enable an engineer to contact and/or monitor a structure at a remote location, using minimal hardware (laptop PC, modem, and an internet account), from virtually anywhere in the world. This paper describes a deployed use of internet-based utilities to 'call up' a remote instrumented structure to monitor and assess its condition and behavior in near real-time. The structure, a 50 m high luminaire tower, has been instrumented both before and after it was retrofitted with fluid viscous dampers to reduce windstorm-induced fatigue demands on its anchorages. With the addition of internet-based hypertext utilities, an engineer can contact and monitor the goings-on at such an instrumented structure at a remote location, from virtually anywhere in the world. In the described case study, the processing workstation calls the PC-based data acquisition system at the remote structure, downloads the current data, processes and analyzes the data and produces summary reports compatible with World Wide Web hypertext based internet browsing software. System results can be used to provide insight into the efficacy of the damper retrofit as well as the accuracy of modeling assumptions used during the design process.

The need for a rapid assessment of the state of critical and conventional civil structures, such as bridges, control centers, airports, and hospitals, among many, has been amply demonstrated during recent natural disasters. Research is underway at Stanford University to develop a state-of-the-art automated damage monitoring system for long term and extreme event monitoring based on both ambient and forced response measurements. Such research requires a multi-disciplinary approach harnessing the talents and expertise of civil, electrical, and mechanical engineering to arrive at a novel hardware and software solution. Recent advances in silicon micro-machining and microprocessor design allow for the economical integration of sensing, processing, and communication components. Coupling these technological advances with parameter identification algorithms allows for the realization of extreme event damage monitoring systems for civil structures. This paper addresses the first steps toward the development of a near real-time damage diagnostic and monitoring system based on structural response to extreme events. Specifically, micro-electro-mechanical- structures (MEMS) and microcontroller embedded systems (MES) are demonstrated to be an effective platform for the measurement and analysis of civil structures. Experimental laboratory tests with small scale model specimens and a preliminary sensor module are used to evaluate hardware and obtain structural response data from input accelerograms. A multi-step analysis procedure employing ordinary least squares (OLS), extended Kalman filtering (EKF), and a substructuring approach is conducted to extract system characteristics of the model. Results from experimental tests and system identification (SI) procedures as well as fundamental system design issues are presented.

This paper presents a qualitative health monitoring technique to be used in real-time damage evaluation of massive complex structures such as bridge joints. The basic principle of the technique is to monitor the structural mechanical impedance which will be changed with the presence of damage. The mechanical impedance variations are monitored by measuring the electrical impedance with a piezoelectric actuator/sensor. This mechanical-electrical impedance relation is due to the electro-mechanical coupling property of piezoelectric materials. This health monitoring technique can be easily adapted to existing structures, since only a small PZT patch is needed, giving the structure the ability to constantly monitor its own structural integrity. This impedance-based method operates at high frequencies (generally above 100 kHz), which enables it to detect incipient type damage, and is not confused by normal operating conditions, vibrations, changes in the structure, or changes in the host external body. This health monitoring technique has only been applied successfully to a variety of light structures. However, the usefulness of the NDE technique for massive structures is uncertain and needs to be investigated. For this purpose, a 500-LB, quarter-scale deck truss bridge joint was built and used in this experimental investigation. The localized sensing area is still observed, but the impedance variations due to incipient damage are slightly different. Nevertheless, by converting the impedance measurements into a scalar damage metric, the real-time implementation of the impedance-based technique has been proven feasible.

An integrated optical fiber sensor system is being developed for highway bridge monitoring. Laboratory small and large scale testing were performed to explore the feasibility of the system. Large scale reinforced concrete beams were fabricated in the laboratory, and artificial flaws in terms of delaminations were simulated in the beams during construction. The sensing system was used to evaluate the effect of these damages on the behavior of the beams.

In 1995, our laboratory fitted a highway bridge near Lausanne (Switzerland) with low- coherence fiber optic deformation sensors. The engineers, who had designed the steel-concrete composite bridge, were interested in the strain distribution inside the concrete slab and in the effects induced by the concrete shrinkage. More than 30 fiber optic deformation sensors, a few vibrating string deformation sensors, thermoelectric couples as well as resistive strain gages were installed in the concrete deck and on the steel girders of this bridge. Three phases of the bridge life were monitored: concreting and thermal shrinkage, load test with heavy trucks and long term deformations. This contribution presents the fiber optic sensor design, the installation technique and the preliminary results obtained on this bridge.

It is necessary to perform structural measurement for identifying the system behavior of a cable-stayed bridge especially during construction since this type of bridge requires precise behavior control during construction. The considerations for deploying a proper measurement or monitoring system are instrumentation, signal processing and information processing. This paper presents a monitoring application considering these three requirements for a steel composite cable-stayed bridge constructed on the Han River in Seoul. An online signal handling scheme for long-term monitoring is also described.

A technique is presented capable of detecting hidden voids underneath a composite patch adhered to concrete structures as reinforcement. The approach is based upon local membrane vibration of a thin panel when it becomes debonded from the host structure. A state-of-the-art laser Doppler vibrometer is used for high-sensitivity, high-speed and noncontact surface vibration detection. The composite repair patch is excited by piezoelectric patch actuators over a wide frequency range to ensure the presence of the local resonance modes in the scanning image. The results show that both boundary and inner disbonds are detectable by this technique, and the location as well as the size of a disbond as small as 0.75 by 1 in2 can be detected when the excitation frequencies are properly selected and the response image is post-processed. The technology has demonstrated its potential for applications such as quality assurance in concrete infrastructure reinforcement and metal structure repair by composite wrapping or patching. It is also appropriate for delamination detection of composite products during manufacturing.

This paper deals with full scale forced modal testing performed on an 80 year old arch bridge over the Aare river in Switzerland. This bridge with a span of 72 m suffers from cracks of its main girders caused by vibration from heavy traffic. The dynamic behaviour of the bridge was determined experimentally with a servo-hydraulic exciter using a band-limited random burst force signal. The responses (accelerations in three orthogonal directions) were measured at 144 points distributed over the bridge. The result of modal testing in the form of a "reference modal/mathematical model" was applied to update an FE-model of the bridge. This updated FE-model of the bridge was used in various parameter studies in order to develop an optimum structural modification of the bridge and immunity to traffic excitation via relocation of structure resonances. In this paper it is shown that full-scaled force modal test is a successful way of obtaining the modal model of large civil engineering structures such as a bridge resulting in a description and understanding of the essential bridge dynamic behaviour. In addition, this technology is the only one which allows the non-linearity of the structure to be controlled at test conditions similar to operating ones. This model may be used to develop optimal control strategies and optimal sensor and actuator placement for active damping of the bridge. The reference mathematical model may be useful in simulating structural failure and different kinds of damage for training an automatic monitoring system based on an artificial neural network to detect, recognise and localise the damages at bridge operating conditions.
Keywords: full-scale modal testing, bridge vibrations testing, civil engineering structures, modal response, servo-hydraulic excitation, finite element model updating, structural modification, damage detection, neural networks, active vibration control

Recent developments in the field of fiber optics suggest that in the near future it may be economically feasible to instrument complex structures with fiber optical strain gages. If a method of damage localization and severity estimation were formulated to accept directly the strain data from such devices, the efficiency and effectiveness of fiber optical strain gages could be significantly enhanced. In this paper a method to locate and size structural damage in complex structures with minimal modal strain parameters is presented. A theory of damage localization and severity estimation is formulated in terms of modal strains. A finite element model (FEM) of the structure to be analyzed is developed. Modal strains are calculated for every member of the undamaged structure. Modal strains are also obtained from the structure with simulated damage or from the actual structure in the field. The damage localization formulation is used to localize damage for a variety of damaged scenarios. Finally, the reliability and accuracy of the method is evaluated.

Aiming at the characteristic of bridge dynamic deflection signal, and the requirement for the frequency characteristics of measuring transducer, after reviewing several measuring methods, the authors put forward a convenient and effective method for measurement of bridge dynamic deflection with seismic low frequency vibration transducers. Signal recovery technique is adopted to compensate the output which is distorted because of the frequency characteristics of seismic transducer. Spectrum extrapolation technique for time-limited signal is employed to reconstruct some low frequency spectral lines which are contaminated seriously by low frequency noise. The work in this paper provides a new way for the measurement of bridge dynamic deflection, which lays the foundation for the measurement and research of bridge spatial vibration excited by a train passing by.

By placing fiber optic gratings in a Sagnac loop a distributed strain sensor may be formed by using the light reflected from the fiber gratings as sources for balanced Michelson and Mach- Zehnder interferometers. In this manner the resulting fiber optic sensor is capable of measuring integrated strain over lengths determined by the fiber grating position, point strain and temperature at the fiber grating locations and localizing and measuring the position of a time varying signal such as an acoustic wave.

A fiber optic sensor has been developed and tested for static and dynamic strain and displacement measurements. The sensor incorporates an extremely simple design, light source, and detector. The sensor utilizes a continuous piece of multimode optical fiber 'tied' into the shape of a figure of eight which functions as an intensity based optical fiber strain or displacement sensor. It is bonded to a body at two points, one on each side of the loops in the eight. The inherent stiffness of the fiber maintains a constant shape for the sensor throughout a wide range of displacements. Since multimode fiber is used in the construction, a simple LED/PIN diode is used for the light source/detector with a basic amplification circuit for each. The experiments have shown that the sensor is simple to construct, has a linear range of approximately 20 mm extension, and responds to frequencies from the quasi-static into the kilohertz range.

The durability of bridges and the ability to maintain their initial structural capacity are topics of growing interest. For a long time, inspection programs have been carried out to follow up a changing state of large structures. To help managers in their decisions, an advanced metrology was introduced in sensitive parts of bridges to assess their structural aptitude. In particular, an abnormal behavior of the structure often causes a load re-distribution on its bearings. If the reaction force is monitored at the bearings, the entire structure can be assessed and its main deficiencies eventually detected. A technology based on using optical fiber sensors was developed and a prototype tested. A system of three or five sensors is incorporated in an external metallic plate put under the bearing. The microbending technique is used and light attenuation is measured. The shape of the plate grooves, containing the fibers, is chosen so that the sensor range and offset can be adapted to the operating load. Bearing reaction real time measurements, introduced into an automatic or semi-automatic processing, provide the structure with a certain 'intelligence.' Theoretical and experimental results are presented.

The problems associated with the application of chloride-based deicing agents to roadways and specifically bridges include chemical pollution and accelerated corrosion of strength members (especially rebar) within the structure. In many instances, local ordinances are attempting to force state agencies to reduce, if not eliminate, the use of these chlorides (typically at the cost of increased driving hazards). With respect to the corrosion aspects of chloride application, cracks that occur in the roadway/bridge pavement allow water to seep into the pavement carrying the chloride to the rebar with the resultant increase in corrosion. In response to this problem, particularly in high roadsalt usage areas, a chloride/water impermeable membrane is placed above the rebar matrix so if/when roadway cracking occurs, the roadsalts won't be able to damage the rebar. Such a membrane is costly -- and the question of its in-service performance is questionable. In a joint effort between the University of Vermont and the Vermont Agency of Transportation, we are developing fiber optic chloride detectors which are capable of being embedded into the rebar-concrete roadway under this membrane. The sensing mechanism relies on spectroscopic analysis of a chemical reaction of chloride and reagents (which have been coated onto the ends of fibers). Laboratory results of these detectors and a usable system configuration are presented.

A novel low-cost fiber optic sensor system for measuring strain in structures has been developed. The system uses low-cost digital electronics and a broad band light emitting diode (LED) to monitor low-attenuation fiber-optic strain gages. This paper introduces this new system. The architecture of the system is discussed, with details of the fiber optic strain sensing elements, the low-cost electronics, and the personal computer (PC) interface to the system. The tradeoffs which allow the low-cost system to be feasible are discussed. The laboratory experiments used to verify system performance are then presented. Finally, a description of a full-scale, outdoor application for monitoring the strain on the surface of a composite pipe segment is described.

The measurement of strain within a compacted soil mass using optical fibers was demonstrated. The sensitivity of the fiber optic sensor was shown to match that of an existing soil strain sensor. Experimental data were gathered by dynamically loading the soil. Data from the optical fibers was processed by a Fabry-Perot interferometer into a localized strain. The data was verified by comparing with data obtained from a LVDT and a commercial soil strain sensor as well as theoretical data obtained from a finite element analysis. The data indicate that fiber optic sensors have the ability to detect viscoelastic soil strains and may be used to measure the permanent deformation of soil.

The object of this research is to assess the feasibility of using the concept of self-healing concretes for structural highway elements such as bridges, and roadway pavements. Our research has concentrated on the material behavior of self-healing cements which internally release adhesive when crack damage occurs. The focus of this research is on the use of self- healing concretes in structural highway members, such as bridges, that may be damaged by dynamic events such as earthquakes, impacts. A following study will investigate the influence of different types of adhesives and release mechanisms in the concrete elements under several load histories, for self-healing of the structural element. In the experimental program, the first set of specimens used typical elements, such as frames containing adhesive loaded fibers. The results were positive. From there we next go on to joints containing several types of adhesives and release mechanisms. These are tested on a small shake table in which actuators, load sensors, and a deflection monitor are mounted on a base. The adhesives have different set times, strength of bond with the matrix, and elastic moduli. The specimens are tested for the effect of adhesive type on deflection, stiffness, and damping of the members.

Direct strain measurement using strain gauges, in the assessment of structural integrity, has been impeded for years by the need for extraordinary surface preparation. New laws and regulations concerning hazardous materials and lead paint removal are likely to further restrict the use of resistance strain gauges. A new acoustic strain measurement system has been tested in the lab to demonstrate the ability of the system to measure strain through paint. These tests confirm the relationship between liftoff (the distance between the transmitter and receiver surfaces and the metal-under-test) and transducer performance and demonstrate that strain can be measured through paint using ultrasonic techniques.

A network of distributed optical Bragg grating sensors is used for monitoring of a full scale laboratory bridge in its pristine and damaged state. Damages consist of a series of cuts that are introduced in an external girder to simulate fracture or fatigue crack of a main load carrying bridge component. The after fracture behavior is described in terms of load path redistribution and strain level changes in the structure.

In this paper, a method is presented to predict the location and magnitude of structural damage in highly nonlinear systems with damping. If a structure exhibits moderate or severe nonlinearities, conventional system damage prediction approaches only yield unsatisfactory results. In particular, techniques based upon modal analysis cannot be utilized. The approach presented here excels in the damage prediction of systems of severe nonlinearities where other methods have difficulties to yield acceptable results. Based upon the first law of thermodynamics, a system of equations is obtained to yield information on the physical properties of the structure regardless of the type of excitation. The equations are directly obtained from dynamic response readings of the system. As opposed to modal analysis techniques, which often require sampling times of excessive length, very short sampling times in the range of only a few seconds will suffice in the presented formulation. The approach is derived for viscously damped structures with polynomial nonlinearities, although the approach is applicable to other types of damping and nonlinearities. The performance of the approach is demonstrated on a numerical example of a multi-span bridge. To demonstrate the versatility of the approach, the method is applied using harmonic as well as impulse excitation. It is shown that the presented approach perfectly predicts damage inherent in a damped nonlinear structure.

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Journal of Applied Remote SensingJournal of Astronomical Telescopes Instruments and SystemsJournal of Biomedical OpticsJournal of Electronic ImagingJournal of Medical ImagingJournal of Micro/Nanolithography, MEMS, and MOEMSJournal of NanophotonicsJournal of Photonics for EnergyNeurophotonicsOptical EngineeringSPIE Reviews